In agriculture and forestry, it is important for efficient growth of plants of interest in a short period to give necessary nutrients in proper amounts when necessary. It is therefore necessary to accurately judge the nutritional statuses of plants and appropriately fertilize them according to the results.
Subjective methods have heretofore been adopted, in which growers judge the nutritional statuses of plants on the basis of their experience and guess and also practice fertilization itself on the basis of the judgment. However, several years are required for cultivating skilled growers capable of such judgment. Also, the judgment disadvantageously differs among growers, in some cases. The further problem of the subjective methods is that whether or not plants are actually provided with their necessary nutrition in necessary amounts when necessary or how much contribution the applied nutrition makes to plant growth cannot be judged objectively.
In recent years, a method has been developed, which comprises analyzing element contents such as nitrogen, phosphorus, and potassium contents in leaves by a Kjeldahl method or an inductively coupled plasma (ICP) analysis method and estimating the nutritional status of the plant from the numeric values to determine the type and amount of a fertilizer (Non Patent Literature 1). This method is based on an objective judgment approach and is theoretically excellent. However, generally, plants largely differ in element contents therein depending on, for example, a difference in growing environments or places, i.e., a difference in stresses of environmental factors under which the plants are placed, even when they are provided with the same fertilizer in the same amount. This method is therefore applicable to only plants cultured in the same environment, i.e., under the same stress. Thus, the method is disadvantageously of limited application and is less practical.
There is also a method for estimating the health of plants using the spectral reflection characteristics of their leaves (Patent Literature 1). This method comprises preparing a database on the reflection spectra of various plants, and comparing the database with sampled data on the spectral reflectances of plants obtained by a high-resolution remote sensing satellite technique to thereby determine the vitality of the plants. However, this method also results in spectra differing depending on different leaves in one individual or a difference in date and time and is therefore applicable to only plants cultured under the same conditions. Thus, the method has disadvantages such as little practicality.
Thus, none of the previously known methods permit accurate and objective judgment of the nutritional status of a plant without being influenced by various stresses.